Prevention of restenosis after coronary angioplasty: towards a molecular approach?

Is there an alternative to systemic anticoagulation, as related to interventional biomedical devices?

To reduce the toxic effects, related clinical problems and complications such as bleeding disorders associated with systemic anticoagulation, it has been hypothesized that by coating the surfaces of medical devices, such as stents, bypass grafts, extracorporeal circuits, guide wires and catheters, there will be a significant reduction in the requirement for systemic anticoagulation or, ideally, it will no longer be necessary. However, current coating processes, even covalent ones, still result in leaching followed by reduced functionality. Alternative anticoagulants and related antiplatelet agents have been used for improvement in terms of reduced restenosis, intimal hyperphasia and device failure. This review focuses on existing heparinization processes, their application in clinical devices and the updated list of alternatives to heparinization in order to obtain a broad overview, it then highlights, in particular, the future possibilities of using heparin and related moieties to tissue engineer scaffolds.

Pharmacological treatment for prevention of restenosis

Coronary artery disease (CAD) is the leading cause of mortality and morbidity among adults in the Western world. Coronary artery bypass grafting and percutaneous coronary interventions (PCI) have gained widespread acceptance for the treatment of symptomatic CAD. There has been an explosive growth worldwide in the utilisation of PCI, such as balloon angioplasty and stenting, which now accounts for over 50% of coronary revascularisation. Despite the popularity of PCI, the problem of recurrent narrowing of the dilated artery (restenosis) continues to vex investigators. In recent years, significant advances have occurred in the understanding of restenosis. Two processes seem to contribute to restenosis: remodelling (vessel size changes) and intimal hyperplasia (vascular smooth muscle cell [VSMC] proliferation and extracellular matrix [ECM] deposition). Despite considerable efforts, pharmacological approaches to decrease restenosis have been largely unsuccessful and the only currently applied modality to reduce the restenosis rate is stenting. However, stenting only prevents remodelling and does not inhibit intimal hyperplasia. Several potential targets for inhibiting restenosis are currently under investigation including platelet activation, the coagulation cascade, VSMC proliferation and migration, and ECM synthesis. In addition, new approaches for local drug therapy, such as drug eluting stents, are currently being evaluated in preclinical and clinical studies. In this article, we critically review the current status of drugs that are being evaluated for restenosis at various stages of development (in vitro, preclinical animal models and human trials).

Anticoagulant and antiplatelet agents: their clinical and device application(s) together with usages to engineer surfaces

An essential aspect of the treatment of patients with cardiovascular disease is the use of anticoagulant and antiplatelet agents for the prevention of further ischaemic events and vascular death resulting from thrombosis. Aspirin and heparin have been the standard therapy for the management of such conditions to date. Recently, numerous more potent platelet inhibitors together with anticoagulant agents have been developed and tested in randomized clinical trials. This article reviews the current state of the art of antiplatelet and anticoagulant therapy in light of its potential clinical efficacy. It then focuses on the usages of these agents in order to improve the performance of clinical devices such as balloon catheters, coronary stents, and femoropopliteal bypass grafting and extra corporeal circuits for cardiopulmonary bypass. The article then goes on to look at the usage of these agents more specifically heparin, heparan, hirudin, and coumarin in the development of more biocompatible scaffolds for tissue engineering.

Surface modification of polyethylene balloon catheters for local drug delivery

Local drug delivery is an attractive approach to the associated problems of percutaneous transluminal coronary angioplasty (PTCA), including arterial injury. The objective of the present research was to deliver a high concentration of a potent anti-thrombin agent, argatroban (ARG), to the vessel wall in order to reduce arterial injury. Local delivery was accomplished by the ionic attachment of drug particles to a modified balloon surface. Surface graft polymerization of ionic monomers to a high-density poly(ethylene) (PE) substrate was performed utilizing ultra-violet (UV) methods. Acrylic acid (AAc) and 2(dimethylamino) ethyl methacrylate (DMAEMA) were successfully grafted onto PE surfaces. Surface grafting was verified by contact angle, X-ray photoelectron spectroscopy, and zeta potential measurements. The amount of ARG adsorbed onto the modified PE surface was highly dependent on the pH of the drug media for both anionic and cationic grafted monomers. The efficacy of local drug delivery to the arterial wall was analyzed using drug-immobilized PE balloon catheters in the rabbit common carotid artery model. High concentrations of ARG (280 nmol/g tissue) were found within the ballooned arterial segment immediately after angioplasty, followed by a decrease after blood flow was restored.

Targeting retroviral vectors to vascular lesions by genetic engineering of the MoMLV gp70 envelope protein

Targeted gene delivery to vascular lesions is a major challenge in the development of gene therapy protocols for cardiovascular diseases. One approach would be to enable retroviral vectors to accumulate at sites of vascular injury and enhance local vector concentration. An early step in wound repair is the adhesion of platelets to collagen exposed from damaged vasculature. Hence, the Moloney murine leukemia virus (MoMLV) envelope (env) protein was engineered to incorporate a high-affinity collagen-binding domain derived from von Willebrand clotting factor, and expressed in Escherichia coli and in mammalian cells. The chimeric env protein bound tightly to collagen, and virions bearing this collagen-binding env protein exhibited viral titers approaching those of virions expressing wild-type (WT) env protein. The chimeric virions were concentrated on collagen matrices, and they retained their infectivity under conditions in which virions bearing WT env protein were washed away. Targeted delivery of the chimeric env protein to injured mouse aorta and selective binding of the collagen-targeted virions to injured rabbit artery were observed. In comparative studies, vascular smooth muscle cell transduction was demonstrated in catheter-injured carotid arteries following infusion of collagen-targeted virions but not of virions bearing WT env protein. Taken together, these observations demonstrate the ability of collagen-targeted virions to localize gene delivery to sites of vascular injury.

Molecular and cellular characterization of baboon C-Raf as a target for antiproliferative effects of antisense oligonucleotides

C-Raf is a an essential member of the growth factor-ras pathway and a target for intervention strategies aimed at blocking cell proliferative responses. Excessive smooth muscle proliferation is considered one cause of the arterial closure in restenosis. Because of the similarity to the human cardiovascular system, a useful current animal model of the disease is a baboon model. As a foundation for animal studies employing antisense oligonucleotides, efforts were made to characterize the molecular and cellular biology of the baboon system. The nucleotide sequence of baboon c-raf cDNA was determined. Antisense phosphorothioate oligonucleotides specific to the 3'-UTR of c-raf mRNA from human and baboon were compared using primary baboon smooth muscle cells in culture. A particular human antisense oligonucleotide, referred to as ISIS 5132, was different by only 2 of 20 bases from the baboon sequence. The corresponding baboon antisense oligonucleotide ISIS 12959, however, was markedly more effective to inhibit c-raf mRNA, protein production, and DNA synthesis, and the results attest to the species specificity of the approach. After antisense treatment, c-raf mRNA levels dropped rapidly, whereas protein levels decreased with a half-life of roughly 24-48 hours, consistent with the antiproliferative effects. The data are discussed with regard to the profile of protein-protein interactions made by C-Raf and with the view that the baboon system closely parallels the human one at the signal transduction level. As this work progressed, a baboon cDNA homolog of a human c-raf-2 pseudogene was isolated, sequenced, and shown to be transcribed into mRNA.